Re: [patch] mm: vmscan: invoke slab shrinkers from shrink_zone()

[Joonsoo Kim]

On Fri, Apr 17, 2015 at 09:17:53AM +1000, Dave Chinner wrote:
> On Thu, Apr 16, 2015 at 10:34:13AM -0400, Johannes Weiner wrote:
> > On Thu, Apr 16, 2015 at 12:57:36PM +0900, Joonsoo Kim wrote:
> > > This causes following success rate regression of phase 1,2 on stress-highalloc
> > > benchmark. The situation of phase 1,2 is that many high order allocations are
> > > requested while many threads do kernel build in parallel.
> > 
> > Yes, the patch made the shrinkers on multi-zone nodes less aggressive.
> > From the changelog:
> > 
> >     This changes kswapd behavior, which used to invoke the shrinkers for each
> >     zone, but with scan ratios gathered from the entire node, resulting in
> >     meaningless pressure quantities on multi-zone nodes.
> > 
> > So the previous code *did* apply more pressure on the shrinkers, but
> > it didn't make any sense.  The number of slab objects to scan for each
> > scanned LRU page depended on how many zones there were in a node, and
> > their relative sizes.  So a node with a large DMA32 and a small Normal
> > would receive vastly different relative slab pressure than a node with
> > only one big zone Normal.  That's not something we should revert to.
> > 
> > If we are too weak on objects compared to LRU pages then we should
> > adjust DEFAULT_SEEKS or individual shrinker settings.
> 
> Now this thread has my attention. Changing shrinker defaults will
> seriously upset the memory balance under load (in unpredictable
> ways) so I really don't think we should even consider changing
> DEFAULT_SEEKS.
> 
> If there's a shrinker imbalance, we need to understand which
> shrinker needs rebalancing, then modify that shrinker's
> configuration and then observe the impact this has on the rest of
> the system. This means looking at variance of the memory footprint
> in steady state, reclaim overshoot and damping rates before steady
> state is acheived, etc.  Balancing multiple shrinkers (especially
> those with dependencies on other caches) under memory
> load is a non-trivial undertaking.
> 
> I don't see any evidence that we have a shrinker imbalance, so I
> really suspect the problem is "shrinkers aren't doing enough work".
> In that case, we need to increase the pressure being generated, not
> start fiddling around with shrinker configurations.

Okay. I agree.

> 
> > If we think our pressure ratio is accurate but we don't reclaim enough
> > compared to our compaction efforts, then any adjustments to improve
> > huge page successrate should come from the allocator/compaction side.
> 
> Right - if compaction is failing, then the problem is more likely
> that it isn't generating enough pressure, and so the shrinkers
> aren't doing the work we are expecting them to do. That's a problem
> with compaction, not the shrinkers...

Yes, I agree that. I will investigate more on compaction.

Thanks.
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